Australia's PALGN rates Halo 3 highly and calls it marginally better than Halo 2, with some of the oft-repeated gripes about ally AI and graphics:

The most bittersweet aspect of the game is the graphics. While they are an improvement from what we saw in the multiplayer beta and they certainly look good, they will be surpassed in the lifetime of the Xbox 360, if they haven't been already. We were pleased with the newfound vibrancy, life and variety in the environments and the design is among the strongest of the series.

[...]

At it's core, Halo has been about the close quarters combat against tough AI. A lot of the enemy patterns have improved, and new behaviours include Grunts sticking plasma grenades to themselves and going kamikaze. The same can't be said for your allies, as their AI is borderline useless. Four will jump on the Scorpion with you; none will make it to the other end.

Call me crazy, but once the marines are on the jumpseats, it's up to the driver's intelligence to keep them alive by shielding them from fire while simultaneously putting them in a position to put fire on the enemy. Their "AI" is irrelevant here. What are they supposed to do, dodge bullets?

PALGN gave the game 9 out of 10 and said that the extra features, like coop play and Forge, really make the game.